148 

 specific inhibition of RNA polymerase II, a critical 

 enzyme to transcription, by amanitin allows low doses of 

 inhibitor to produce a significant cytotoxic effect. This 

 is analogous to the inhibition produced by diphtheria 

 toxin which is extremely toxic because of the ADP ribo- 

 sylation of elongation factor 2 resulting in specific inhi- 

 bition of protein synthesis. Diphtheria toxin however, 

 is macromolecular in nature, and the procedures used for 

 conjugation tend to produce conjugates of ill defined 

 molecular composition due to cross-linking of the diph- 

 theria toxin with itself and/or cross-linking of the car- 

 rier protein. Alpha-amanitin conjugates circumvent the 

 problem of nonhomogeneity by virtue of the well-defined 

 mechanism of conjugation via a single available site on 

 the amanitin derivative for conjugation to proteins. 

 Furthermore, the coupling conditions defined resulted in 

 insignificant levels of intermolecular protein cross- 

 linking as evidenced from the SDS-PAGE data. 



Conjugates of inhibitors that interact with DNA 

 (daunomycin, 5-FUDR, chlorambucil) tend to be more homo- 

 geneous in composition like oc-amanitin conjugates as a 

 consequence of having a limited number of points on a 

 small molecule available for conjugation to proteins. 

 However, their toxicity is a result of interaction with 

 DNA and thus requires a much larger dose of inhibitor to 

 achieve significant cytotoxicity than would be required 

 for an inhibitor that acts on an enzyme essential to 



